It's a generic, uninteresting design, but that's good for our purposes...it's easier to see how each little change makes a difference, I think.
So here are the factors I used to change the appearance of each drawing:
1. The size and angularity of the neck and shoulders: Babies start out with thick necks, but then by the time we're 5 or 6 our necks seem to look pretty skinny (at least I've found it seems to look that way in drawings). Our shoulders, obviously, start out small and get broader and broader as we get older (this is more apparent in men than in women, of course). Also I made a conscious effort to make his shoulders start out rounded and get more angular as he ages. If I'd kept going, drawing this guy into old age, I would have made the shoulders start to shrink (and get more rounded again) as he aged.
2. The relation of the mouth to the chin: I simply made the mouth get further away from the chin as he ages, to give a feeling of a jaw that develops as he gets older. Also I made the jawline stronger and more angular with each successive age.
3. The amount of upturn in the nose: I made it turned up more in the younger drawings, and made it less so over time.
4. The size of the forehead: We have a bigger forehead when we're young, and it diminishes proportionally over time.
5. The chubbiness of the cheeks: most of us have less fat in our cheeks as we age.
These aren't the best drawings, admittedly, but hopefully all this makes sense and people will find it helpful. I don't have much experience at drawing people at different ages and it's something I'm trying to get better at doing.
When I was a younger artist, I always assumed that great artists with lots of experience sat down and told young artists just starting out how to do things like draw people of different ages. It's funny, that's not at all how things work and I can't say I've ever been told any of this stuff, it's all just observations I've made (and of course I've appreciated the other stuff people have written down in other drawing books...see below). If you're lucky enough to have been given some other pearls of wisdom (or observed other things you'd like to share), by all means leave us a comment and let us all know!